1. Field of the Invention
The invention relates to a powertrain control apparatus and method. More particularly, the invention relates to a powertrain that includes a lock-up clutch that connects an engine directly to an automatic transmission.
2. Field of the Invention
A conventional automatic transmission is connected to an engine via a fluid coupling such as a torque converter. The torque converter transmits the driving force to the transmission through the fluid (e.g., oil) that circulates in the torque converter. Therefore, the rotational speed of the input shaft of the torque converter differs from the rotational speed of the output shaft of the torque converter. This may decrease the efficiency of transmitting the driving force. Accordingly, a lock-up clutch that mechanically connects the input shaft to the output shaft of the torque converter is generally provided.
To improve fuel efficiency, when the vehicle speed is equal to or above a predetermined speed while the vehicle is decelerating, the fuel supply is cut off, that is, fuel injection is stopped. If the vehicle speed decreases to the predetermined speed when the fuel supply is cut off, the fuel injection resumes (i.e., the fuel supply resumes). After the fuel supply resumes, engine speed increases. Therefore, if the lock-up clutch is engaged when the fuel injection resumes, a shock may occur, and drivability may deteriorate. Accordingly, the powertrain is controlled so that the lock-up clutch is disengaged when the fuel supply resumes.
Japanese Patent Application Publication No. 2005-114069 (JP-A-2005-114069) describes a lock-up clutch control apparatus for a vehicle. When the lock-up clutch is disengaged, the control apparatus sharply decreases the hydraulic pressure applied to the lock-up clutch. The control apparatus decreases the hydraulic pressure from a value at which the lock-up clutch is engaged to a value at which the process of disengaging the lock-up clutch starts (hereinafter, this value will be referred to as “disengagement initial value”). Then, the control apparatus gradually decreases the hydraulic pressure to a pressure where the lock-up clutch is completely disengaged. The control apparatus described in JP-A-2005-114069 includes a disengagement portion, and an initial-pressure setting portion. When the lock-up clutch is disengaged while the vehicle is decelerating, the disengagement portion outputs an instruction to sharply decrease the hydraulic pressure from the value at which the lock-up clutch is engaged to the disengagement initial value, and then the hydraulic pressure gradually decreases to the value at which the lock-up clutch is completely disengaged. The initial-pressure setting portion sets the disengagement initial value according to the inertial torque of the engine when the vehicle is decelerating.
In the control apparatus described in JP-A-2005-114069, if the lock-up clutch is disengaged while the vehicle is decelerating, the disengagement portion sharply decreases the hydraulic pressure from the value at which the lock-up clutch is engaged to the disengagement initial value. The initial-pressure setting portion sets the disengagement initial value according to a parameter relating to the inertial torque of the engine or the inertial torque of the engine crankshaft system when the vehicle is decelerating. For example, the disengagement initial value is set according to the deceleration of the vehicle, or the rate of change in the engine speed when the vehicle is decelerating, or a parameter relating to the deceleration or the rate of change in the engine speed. This reduces variations of the time at which the lock-up clutch is actually disengaged if the torque capacity required for the lock-up clutch varies due to variations in the inertial torque of the engine crankshaft system. Thus, the time required to actually disengage the lock-up clutch is made constant, regardless of the inertial torque of the engine crankshaft system, which may vary depending on the decelerating state of the vehicle, that is, regardless of the parameter relating to the inertial torque. As a result, it is possible to reduce the variations of the magnitude of any shock that may occur when the lock-up clutch is disengaged. Thus, drivability is appropriately improved.
If the lock-up clutch is disengaged when the fuel supply is resumed, if the combustibility of fuel is low, the engine speed may not increase, and may instead decrease. If the engine speed greatly decreases, the engine may stall. However, JP-A-2005-114069 does not consider the possibility that the engine speed decreases if the combustibility of fuel is low. Therefore, according to the described lock-up clutch control apparatus, the engine may stall when the lock-up clutch is disengaged.